1. Field of the Inparticlerial Application
This invention relates to a gas purge unit for a portable container which is suitably used in a clean room employed for instance in the manufacture of semiconductors.
2. Description of the Prior Art
Semiconductors are manufactured, for instance, in a clean room, the atmosphere in which has been purified. In the manufacture, in order to eliminate the difficulty that particles stick on the semiconductor wafers, the semiconductor wafers are conveyed as follows: The semiconductor wafers are set in a wafer cassette, and the wafer cassette is loaded in a closed container. That is, the closed container accommodating the semiconductor wafers is conveyed.
Recently, semiconductors have been improved in performance, and increased in the degree of integration. Therefore, pollution of the surfaces of semiconductor wafers, that is, formation of native oxide film on them by oxygen and moisture in the air has been a serious problem in one of the semiconductor manufacturing steps. In order to prevent the pollution, a gas purge method has been employed in which the atmosphere in the closed container is replaced with a gas inactive with the wafers, such as nitrogen gas or refined dry air.
On the other hand, in a semiconductor manufacturing equipment such as a CVD/diffusion equipment, a gas purge operation using a gas such as nitrogen gas has been employed because of the following reason. In the CVD/diffusion equipment, a semiconductor film is formed in the furnace at high temperature. If air (oxygen) is drawn in the furnace when the wafer is conveyed into the furnace, then an unwanted film is formed, thus decreasing the performance of the resultant semiconductor. In this connection, it should be noted that the gas purge operation is carried out only when the wafer is moved from the closed container to the manufacturing equipment.
Purging the closed container of gas in the operation of the CVD/diffusion equipment will be described in more detail.
FIG. 3 shows the conveyance of the wafers from the closed container to the CVD furnace. The inside of the manufacturing equipment 201 has been filled with nitrogen gas, while the closed container 101 conveyed to it has been filled with air (oxygen). Therefore, after the air in the closed container 101 is replaced with nitrogen gas, the wafer cassette 102 is taken out of the container 101. The wafer cassette 102 is set on a shelf 202 in the equipment by a cassette handling device 207. Under this condition, a wafer placing device 203 operates to take the wafers out of the wafer cassette one (or more than one) at a time, and place them in a quartz boat 204. The quartz boat 204 with the wafers is lifted to the CVD furnace by a boat lifting device 205, where they are loaded in a quartz tube 206.
As compared to the above-described wafer portable operation, the invention concerns the process of replacing the oxygen gas in the closed container with nitrogen gas, and a method of preventing the oxygen gas in the closed container from flowing into the equipment. As shown in FIG. 3, the inside of the equipment is partitioned with a port skirt 103 at one corner, and in addition to the port skirt 103, a coil spring 104 for lifting a port door 105, a bellows 106, an elevator 107, etc. are provided, and a nitrogen gas inlet 108 and a nitrogen gas outlet 109 are provided in such a manner that they penetrate a port plate 110 radially. Additonally, reference numeral 111 designates seal rings; and 112, a lid. In the equipment thus constructed, the air therein is replaced with a nitrogen gas as follows:
(1) When the closed container 101 is set on the port 110, a sensor (not shown) operates to activate the elevator 107, so that the bottom lid of the closed container 101 is disengaged from the container body, and moved downwardly with the port door 105.
(2) As a result, the port skirt 103 and the container 101 form a closed space, and the nitrogen gas inlet and outlet are communicated with the closed space A; i.e., the inside of the container 101. Under this condition, the nitrogen gas is forcibly supplied into the container 101, so that the air in the container 101 is replaced with the nitrogen gas.
(3) Thereafter, the port door 105 is lowered to a predetermined wafer cassette 102 transferring position, where the wafer cassette 102 is handled by the cassette handling device 207 of the manufacturing equipment 201.
In the case where the closed container 101 filled with the inert gas is used for conveyance and storage of the wafer cassette 102, sometimes the nitrogen gas leaks from the container 101 during conveyance or storage, so that its concentration becomes lower than a predetermined value.
In this case, since the gas purge operation is carried out only by the above-described manufacturing equipment, the container 101 is returned to the equipment during conveyance or storage so that it is purged of gas. This is not economical, and makes the management of the close container 101 troublesome.
As was described above, the gas purge mechanism of the manufacturing equipment 201 needs intricate means such as the skirt 103, the bellows 106, the coil spring 104, the seals 111, etc. in order to prevent the air (oxygen) or contaminated gas in the closed container 101 from flowing into the equipment 201 during the gas purge operation. On the other hand, the gas purge mechanism is not suitable as means for maintaining the closed container 101 filled with nitrogen gas, because its original purpose of use is to convey the wafer cassette 102 into the equipment, and the mechanism is unavoidably bulky because of its large vertical stroke.